The present invention relates to alloy powder for rare earth metal-iron group metal targets for magneto-optical recording media, a rare earth metal-iron group metal target and methods of producing them.
Recently, vigorous development has been conducted to provide a magneto-optical disk, which is a rewritable, high-density recording medium, formed by sputtering a thin layer composed of rare earth metals and iron group metals in a desired composition on a glass or plastic disk substrate. Targets used in the sputtering are conventionally produced by various methods as follows:
(1) Melting a rare earth metal-iron group metal alloy having a desired composition in vacuum or in an inert gas atmosphere and then casting it into a desired shape similarly in vacuum or in an inert gas atmosphere to provide a target ("Nikkei New Material," p.61, Nov. 24, 1986):
(2) Melting and casting an alloy having a desired composition in vacuum or in an inert gas atmosphere into an ingot, and pulverizing it into powder which is then sintered under pressure (Japanese Patent Laid-Open No.61-91336):
(3) Mixing rare earth metal powder with iron group metal powder in a desired proportion, and sintering the mixed powder under pressure at a temperature lower than a liquid phase-appearing temperature (Japanese Patent Laid-Open No.61-99640); and
(4) Preparing iron group metal-rare earth metal alloy powder containing the iron group metal in an amount smaller than the desired value by melting and pulverizing, mixing the above alloy powder with iron group metal powder in a desired composition, pressing the powder mixture into a desired shape and then sintering it (Japanese Patent Laid-Open No.60-230903).
However, all of the above methods suffer from various problems. Specifically, when a target is produced by the method (1), the following problems take place:
(a) Additive elements are likely to be segregated in the casting operation, making it difficult to provide a homogeneous target. PA1 (b) Since extremely brittle intermetallic compounds are formed in the rare earth metal-iron group metal alloy, it is difficult to utilize a structure-homogenizing process such as forging. Therefore, it is impossible to eliminate defects such as holes or voids formed in the casting process. PA1 (c) Since it is brittle, chipping and cracking are highly likely to take place in the formation of a target, meaning that mechanial working is extremely difficult. In addition, thermal stress applied in a bonding or sputtering operation tends to cause cracking of the target. PA1 (d) When a target produced by method (1) is used for sputtering to form a thin layer for a magneto-optical disk, the thin layer tends to have a composition which is richer in an iron group metal by nearly 7-10 atomic % than the desired composition, making it difficult to control the composition of the resulting thin film. PA1 (a) Since rare earth metals have a strong affinity for oxygen, they are likely to be oxidized in the course of pulverization, powder handling and pressure sintering, making it difficult to provide low-oxygen sintered bodies. If a target produced by this method is used for sputtering, the oxygen contained in the target material is introduced into the resulting thin film, thereby selectively oxidizing the rare earth metals and so deteriorating the magnetic properties of the thin film, particularly the coercive force Hc thereof. PA1 (b) In the early stage of sputtering, a rare earth metal phase portion is more preferentially sputtered than an iron group metal phase portion, and only in the later stage is equilibrium achieved in a sputtering rate between the iron group metal phase and the rare earth metal phase. Accordingly, presputtering for a long period of time is needed until the composition of the thin layer being formed becomes stable, making the preparation of the target less efficient. PA1 (a) Although the time necessary for presputtering until the composition of the thin layer being formed becomes stable is shorter than in the case of a target produced by the above method (3), a large difference in composition exists between the target desired and the thin layer formed and also the thin layer suffers from large unevenness of composition in the plane thereof. PA1 (b) A target with high density cannot be obtained merely by pressing a powder mixture and sintering the resulting green body in an inert gas or in vacuum; a target thus produced has voids which cause abnormal discharge in the process of sputtering. PA1 (c) With respect to machinability, the target produced by the method (4) is relatively improved compared to those produced by the methods (1) and (2), but substantially all rare earth metals contained in the material are formed into intermetallic compounds with iron group metals, so that cracking and chipping are highly likely to take place in the process of mechanical working, for instance, by a lathe. PA1 (a) It is reported by "Summary of Lectures in 10th Academic Meeting of Japan Applied Magnetics Association," pp.128-129, 1986 that the difference in composion between a target used and a thin layer being formed can be eliminated by properly adjusting the the proportions of rare earth metals, iron group metals and rare earth metal-iron group metal intermetallic compounds. Nevertheless, with a target whose structure is controlled by a heat treatment as in Japanese Patent Laid-Open No.62-70550, the following problems occur: PA1 (b) From the aspect of mechanical strength, brittle rare earth metal-iron group metal intermetallic compounds existing around the iron group metals should be as thin as possible, but they undesirably tend to become thick by abnormal growth as mentioned above. PA1 (c) A target produced by sintering a mixture of rare earth metal powder and iron group metal powder inherently tends to have a high oxygen content, and also due to heat treatment the final product is further likely to have a high oxygen content, deteriorating the characteristics of the thin layer formed therefrom. PA1 (d) A sintered body is liable to warping and bending the course of cooling from a heat treatment temperature to room temperature.
When a target is produced according to the method (2) a homogeneous target can be obtained, but the same problems as those of targets produced by the above method (1) exist because alloy particles per se are composed of brittle intermetallic compounds.
With respect to the method (3), the following problems exist:
Last, when the above method (4) is used, specifically when iron group metal-rare earth metal alloy powder is mixed with about 1-10 weight % of an iron group metal to provide a powder mixture having a desired composition which is then pressed and sintered, as shown in Example of Japanese Patent Laid-Open No.60-230903, the following problems take place:
Japanese Patent Laid-Open No.62-70550 discloses a target substantially free from all problems inherent to the above-mentioned conventional technologies, and has a fine mixed structure in which intermetallic compounds of rare earth metals and iron group metals are bonded with iron group metals by sintering. More specifically, this target having the above structure is obtained by mixing rare earth metal powder with iron group metal powder in a desired proportion, subjecting the resulting mixture to pressure sintering at a temperature lower than a liquid phase-appearing temperature to form a sintered body composed of rare earth metals and iron group metals, and then heating it at a temperature equal to or higher than the liquid phase-appearing temperature for a short period of time to convert the rare earth metals in the sintered body into intermetallic compounds.
With this target, the rare earth metals take a shorter time to sputter when they are converted into metallic compounds than when they are not chemically bonded, and so their sputtering rate becomes closer to that of the iron group metals. As a result, the problem of unstableness in compostion of a sputtered thin layer which has been pointed out conventionally can be solved, and the target has strength which sufficiently ensures mechanical workability due to the iron group metals contained therein.
It has been found through investigations by the inventors, however, that the technology of Japanese Patent Laid-Open No.62-70550 still suffers from the following problems:
(i) Intermetallic compounds around the iron group metals grow abnormally during the heat treatment: PA2 (ii) There is a eutectic alloy phase composed of an .alpha.-phase of rare earth metals and a rare earth metal-iron group metal intermetallic compound phase in addition to an iron group metal phase and an intermetallic compound phase as shown in FIG. 6C of Japanese Patent Laid-Open No.62-70550, but it is extremely difficult to control the amounts of the .alpha.-phase of rare earth metals and the rare earth metal-iron group metal intermetallic compounds precipitated in this eutectic alloy phase; and PA2 (iii) Unevenness in structure is likely to appear. Therefore, the sputtering with the above target causes the following problems; PA2 (ii) Even with the same target composition, the thin layer composition and characteristics vary from one heat treatment lot to another, meaning that there is large non-uniformity among lots.
(i) If extended sputtering is required, unevenness in the composition and characteristics of the thin layer formed tends to appear; and